Physical Nature of Structure and Properties Degradation of Rail Surface after Long Term Operation

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By methods of optical, scanning and transmission electron diffraction microscopy and microhardness and tribology parameters measurement the changes regularities of structure-phase states, defect substructure of rails surface after the long term operation (passed tonnage of gross weight 500 and 1000 mln. tons) were established. It is shown that the wear rate increases in 3 and 3.4 times after passed tonnage of gross weight 500 and 1000 mln. tons, accordingly, and the friction coefficient decreases in 1.4 and 1.1 times. The cementite plates are destroying absolutely and cementite particles of around form with the sizes 10-50 nm are forming after passed tonnage 500 mln tons. The appearance of dynamical recrystallization initial stages is marked after the passed tonnage 1000 mln tons. It is shown that the operation of steel rails is accompanied by full fractures in surface layers with lamellar pearlite grains and the formation of ferrite–carbide mixtures with nanosized particles. The deformation of steel increases the densities of scalar and excess dislocations, the curvature–torsion values of the crystal lattice, and the amplitudes of internal stress fields. The possible mechanisms of established regularities are discussed. It is noted that two competitive processes can take place during rails long term operation: 1. Process of cutting of cementite particles followed by their carrying out into the volume of ferrite grains or plates (in the structure of pearlite). 2. Process of cutting, the subsequent dissolution of cementite particles, transition of carbon atoms to dislocations (into Cottrell atmospheres), transition of carbon atoms by dislocations into volume of ferrite grains or plates followed by repeat formation of nanosize cementite particles.

Abstract: The paper presents an overview of a number of unusual phase transformations which take
place in pearlitic steels in conditions of the severe deformation, i.e. combination of high pressure
and strong shear strain. Strain-induced cementite dissolution is a well-documented phenomenon,
which occurs during cold plastic deformation of pearlitic steels. Recently new results which can
shed additional light on the mechanisms of this process were obtained thanks to 3DAP and HRTEM
investigations of pearlitic steel deformed by high pressure torsion (HPT). It was shown that the
process of cementite decomposition starts by carbon depletion from the carbides, which indicates
that the deviation of cementite’s chemical composition from the stoichiometric is the main reason
for thermodynamic destabilisation of cementite during plastic deformation. Important results were
obtained regarding the distribution of released carbon atoms in ferrite. It was experimentally
confirmed that carbon segregates to the dislocations and grain boundaries of nanocrystalline ferrite.
Another unusual phase transformation taking place in nanocrystalline pearlitic steel during room
temperature HPT is a stress induced α→γ transformation, which never occurs during conventional
deformation of coarse grained iron and carbon steels. It was concluded that this occurred due to a
reverse martensitic transformation. The atomistic mechanism and the thermodynamics of the
transformation, as well as issues related to the stability of the reverted austenite will be discussed.

Abstract: Eutectoid steels present a wide range of interesting mechanical properties (high strength, wear resistance, ductility and toughness) and could be a cheaper alternative to high strength low-alloyed steels (HSLA) in applications where weldability is not a critical requirement. The mechanical properties of pearlite are mainly dictated by the interlamellar spacing and the spheroidization of cementite. In this work, the spheroidization kinetics during annealing of a fully pearlitic steel produced in an electric arc furnace (EAF) is investigated. More specifically, the influence of a prior cold deformation and of the interlamellar spacing is studied using image analysis and hardness tests. It is shown that spheroidization is faster in fine pearlite than in coarse pearlite. Prior cold deformation strongly accelerates the spheroidization kinetics in fine and coarse pearlite. The tensile properties corresponding to different pearlite microstructure were measured and are compared to the hardness evolution during annealing.

Abstract: The ultra-microduplex structure was fabricated in the high carbon steel with a fully pearlitic structure after severe plastic deformation. The sizes of ferrite grains and cementite particles were about 0.4 μm and 0.1~0.2 μm, respectively. The mechanical properties of the ultra-microduplex structure were investigated using mini-tensile tests and the morphologies of fracture surfaces were observed with scanning electron microscopy (SEM). The results show that the tensile strength of the ultra-microduplex structure and the lamellar pearlite are almost at the same level, but after warm deformation, the yield strength was obviously increased and correspondingly, the elongation and the reduction of area were 19.2%, 32.1%, respectively, which are markedly higher than those of the lamellar pearlite. The tensile fracture of the ultra-microduplex structure is typical ductile fracture, however the fracture of original lamellar pearlite appears a mixture of cleavage fractures and quasi cleavage fractures.

Abstract: The aim of this research is to investigate the effect of Cr and Al (strong ferrite formers) on the strain-induced γ-to-pearlite transformation in eutectoid steels. The microstructure evolution during the hot deformation of three eutectoid steel grades was investigated using hot torsion testing. More specifically, the steels were deformed to strain levels varying from ε = 0,5 to ε = 1,5 at their specific Ar1 temperature. Hot deformation of the undercooled austenite leads to strain-induced γ-to-pearlite transformation and to the almost instantaneous spheroidization of the formed carbides. The corresponding microstructures consist of submicronic cementite particles and ferritic grains that are 1-5 μm in size. It is shown that 1,5% Cr addition and 0,5% Al addition increase the equilibrium transformation temperature but slower significantly the kinetics of the strain-induced transformation and consequently reduce the kinetics of cementite spheroidization and of ferrite recrystallization.

Abstract: The cold formability of ferritic-pearlitic steels is one of the base parameters for material choice for different forming parts. One of the key factors is the pearlite morphology, which is strongly dependent on chemical composition and previous treatment history. The carbides in pearlite occur mainly in the lamellar form. One of the ways of improving the ductility along with formability is the change of lamellar carbides to globular carbides. This can be conventionally done by soft annealing, which is characterised by long processing times and high energy costs. This paper presents a new processing modification which can lead on the one hand to significant shortening of carbide spheroidization times and on the other hand to intensive refinement of grain size even for low-carbon steels. Low temperature thermomechanical treatment with variation of the heating temperature around Ac1 and incremental deformation was examined on low carbon plain RSt-32 steel. After the thermomechanical treatment conditions were optimized, the refinement of the ferritic grains from an initial 30 μm to circa 5 μm took place, and the time necessary for carbide spheroidization was shortened from several hours to several seconds.